The disclosed subject matter relates generally to telecommunications and, more particularly, to a method and apparatus for controlling ringing voltage.
In communications systems, particularly telephony, it is common practice to transmit signals between a subscriber station and a central switching office via a two-wire bidirectional communication channel. A line card generally connects the subscriber station to the central switching office. A line card typically includes at least one subscriber line interface circuit (SLIC) as well as a subscriber line audio-processing circuit (SLAC). The functions of the line card include range from supplying talk battery to performing wake-up sequences of circuits to allow communications to take place.
Subscriber line interface circuits (SLICs) have been developed to provide an interface between a low voltage signal path in a telephone central office and a high-voltage telephone subscriber line. The SLIC provides functions such as off hook detection, ringing signal generation, and battery feed to the subscriber line. The subscriber line consists of a telephone transmission line, including two conductors referred to as A and B or tip and ring, and the subscriber telephone equipment coupled across the tip and ring conductors (i.e., the load). The subscriber line and the subscriber telephone equipment are also referred to as a subscriber loop.
The SLIC provides power from the telephone central office to the subscriber line in response to a received battery voltage. The battery voltage is a DC voltage supplied to the SLIC to power the SLIC and the subscriber line. A typical value of the battery voltage is −48 VDC. The battery voltage has a value generally in the range −20 to −60 VDC. The SLIC supplies a DC current at the battery voltage to the subscriber line. Superimposed on the DC current are AC signals of audio frequency by which information is conveyed between the subscriber and the central office. The battery voltage is generated at the central office, either by a depletable energy storage device such as a battery or by a DC generator, for supply to the SLIC. In a central office, one battery or DC generator supplies the battery voltage to many SLICs and their associated subscriber loops.
In many modern applications, a SLIC is located remote from the central office, relatively close to the subscriber telephone equipment and coupled to the subscriber telephone equipment by a relatively short subscriber line. For example, in fiber in the loop (FITL) applications, the SLIC is located in the same city neighborhood as the subscriber telephone equipment and is coupled to the subscriber telephone equipment by tip and ring conductors no more than a few hundred feet in length. The SLIC or an associated circuit receives optical signals from the central office over an optical fiber and converts the optical signals to AC electrical signals. In response to the electrical signals, the SLIC supplies AC signals of audio frequency, along with DC power from the battery, to the subscriber line. In such applications, where the SLIC and battery are remote from the central office, one battery or battery voltage generator may supply the battery voltage to only one or a few SLICs and their associated subscriber loops.
In applications where a SLIC is used in a central office and there are short lines, the loop voltage drop is low, resulting in a high voltage drop and consequently high power dissipation in the SLIC. In a dense environment of such devices, related heating can cause failures. In addition, with SLICs having resistive feed characteristics used in short lines, current is higher, further compounding this problem.
Newer generation chipsets are designed to operate in high density line card applications. The limited board space available in such line cards constrains the size of the package that is available for the devices like the SLIC and the SLAC. Another consideration that also reduces the size of the package is the desire to have lower per line cost for the line card. In general the smaller packages do not present a problem when the associated device does not generate significant heat. However for silicon devices, like the SLIC, that interface with the telephone line, the reduced silicon die size and the reduced package size coupled with the requirement of having to drive out heavy duty ringing signals to the telephony equipment present a challenging design problem with respect to heat generation.
In general, the ringing state is a challenging state in terms of power dissipation for the SLIC device. In a ringing state, the SLIC makes use of all of the available battery sources to drive out the maximum ringing signal. The rationale behind driving out the maximum possible ringing is that the SLIC needs to apply upwards of 40 Vrms for the ringing signal at the longest loop (>1900 ohms) across a ringer load that is at least 5 REN. REN stands for Ringer Equivalent Number. It is a measurement of how much ringing power certain telephone equipment takes. REN numbers are used in the United States to designate how many pieces of telephony equipment can be connected to the same subscriber line and still get them ringing properly.
When the same ringing voltage specifications that were derived to meet the specifications for the long loops are applied to heavy REN loads (e.g., 5C4A or 10K in parallel with 8 uF) with little or no loop in between the SLIC and the load, substantial SLIC power dissipation is present due to the smaller magnitude of the load impedance and due to the phase shifted load current with respect to the drive voltage. The phase shifted load current reaches its peak value when the load voltage is minimum and hence the drop across the SLIC is maximum. This condition results in significant SLIC power dissipation. This power dissipation is illustrated in the following equation:PSLIC=PBattery−PLoad+PCurSense,  (1)where PSLIC is the average power dissipated in the SLIC, PBattery is the power drawn from the battery, PLoad is the power dissipated in the all of the load (i.e., telephony equipment and loop), and PCurSense is the power dissipated in the current sense resistors of the SLIC.
Expanding the individual power components yields:
                                          P            SLIC                    =                                                    2                ·                                  V                  Bat                                ·                                  V                  Ring                                                            π                ·                                                                        Z                    OVL                                                                                          -                                                            cos                  ⁡                                      (                                          ∠                      ⁢                                                                                          ⁢                                              Z                        OVL                                                              )                                                  ·                                  V                  Ring                  2                                                            2                ·                                                                        Z                    OVL                                                                                          +                                                            R                  CurSense                                ·                                  V                  Ring                  2                                                            2                ·                                                                                                Z                      OVL                                                                            2                                                                    ,                            (        2        )            where VBat is equal to the total battery voltage applied across the SLIC (VBP−VBH), VRing is the peak ringing voltage generated by the SLIC, ZOVL is the overall complex load including the internal current sense resistors of the SLIC, fuse resistors, loop resistance, and the REN load, RCurSense is the sum of the current sense resistors used in the SLIC. Equation (2) is derived for a sinusoidal ringing waveform without any DC bias during ringing.
The first equation indicates that the power dissipated in the SLIC should be equal to power drawn from the battery, less the power that is delivered to the load. The third term comes in to play because the SLIC has small current sense resistors that are used to measure the load current. Since these resistors also dissipate power, the SLIC power includes this term. The second equation indicates various power components in terms the applied battery voltages, generated ringing voltages, and the overall load impedance and the SLIC current sense resistor. Note that the PLoad component of the power would be zero if the phase shift introduced by the load circuit is 90 degrees. This condition is approached for heavily reactive loads like 10K∥8 uF and 5C4A REN loads. For these cases, the SLIC power increases as the generated ringing signal is increased. For resistive types of REN loads, the load power would have zero degree phase shift and hence more power would be delivered to the load, thus decreasing the power dissipated in the SLIC.
As an example case, assuming VBat=VBP−VBH=90V−(−60V)=150V, ZOVL=RCurSense (36 ohms)+2*RFuse(50 ohms)+0 ohm loop+10K∥8 uF Pretrip REN load, at 20 Hz ringing frequency (i.e., ZOVL=233.98−984.97j), VRing=130V peak ringing with no DC bias, the power dissipated in the SLIC is PSLIC=12.2623−1.929+0.2968=10.63 W. Hence, the SLIC is being asked to dissipate more than 10 W of power. Such a power dissipation would generate considerable amount of heat from the SLIC device. To remove the resulting heat generated from the SLIC at this power would require additional surface area for better thermal conductivity, thereby reducing the density of the line card. This condition works against the design objectives of lower cost and reduced overall board size. Note that similar SLIC power dissipation conditions would arise even for the 5C4A types of REN load as well under the same conditions.
The ring-trip conditions present another challenge to the SLIC in terms of SLIC power dissipation. Some devices have a hardware analog current limit circuit that limits the loop current (e.g., around 100 mA). Such a mechanism is employed to prevent the transformer magnetic core saturation in the integrated voice and data (IVD) splitters and hence to prevent cyclic redundancy check (CRC) errors in the IVD and excessive current draw from the power supply connected to the line car. The current limiting circuit acts when the subscriber goes off-hook while ringing or during typical DC feed conditions. The hardware current limit circuit limits the loop current even before the firmware has had a chance to react to detect the ring-trip. When the subscriber goes off-hook while ringing the phone, in a short loop application, most likely the hardware current limit circuit will limit the load current. The hardware current limit circuit accomplishes limiting the loop current by reducing the voltage applied across tip/ring leads. Assuming the system has 136 ohms of fixed resistance (fuse and current sense resistors in the SLIC) and a 200 ohm off-hook resistance, the current limit circuit would limit the voltage across this load to the assumed current limit of 100 mA times 336 ohms volts, unless of course the voltage across the load is smaller than this voltage. So, while ringing, when the user goes off-hook the voltage across the load would be a clipped sine wave whose magnitude at the generator has a peak voltage of 33.6V. If the battery applied made use of during ringing is 150V, the remaining 150V−33.6V+3.6V=120V is dropped across the SLIC. This condition implies a SLIC power of 12 W under current limit conditions. Again, this level of SLIC power dissipation works against the design objectives of lower cost and reduced overall board size.
This section of this document is intended to introduce various aspects of art that may be related to various aspects of the disclosed subject matter described and/or claimed below. This section provides background information to facilitate a better understanding of the various aspects of the disclosed subject matter. It should be understood that the statements in this section of this document are to be read in this light, and not as admissions of prior art. The disclosed subject matter is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above.